1. Field of the Invention
The present invention relates to a FM (Frequency Modulation) transmitting antenna and in particular, to a built-in FM transmitting antenna applied to a mobile device.
2. Description of Related Art
Frequency modulation broadcasting system (FM system) is a broadcasting system utilizing a frequency modulation technique to transmit audio signals. Compared with amplitude modulation (AM) broadcasting system, the FM system has advantages of constant envelope, capabilities of anti-nonlinear-distortion and anti-fading-effect, etc. Thus, the FM system has better signal quality and is capable of transmitting stereo signals, which includes left and right channel signals.
A receiver (ex. radio) of the FM system receives FM signals through an antenna, and outputs audio signals after frequency-down-conversion, demodulation, and related operations. Therefore, the main factor of audio quality outputted by the radio depends on reception efficiency of the antenna. In the related art, the majority of FM antennas are external antennas, and rod monopole antennas and dipole antennas covered with PolyEthylen (PE) are representatives.
Please refer to FIG. 1 and FIG. 2, which are schematic diagrams of a rod monopole antenna 1a and a PE dipole antenna 2a. The rod monopole antenna 1a is commonly fixed on a corner of a housing of a portable stereo or a handy radio. When a user wants to listen to broadcasting programs, the user needs to draw out the rod monopole antenna 1a to a specific length. When the rod monopole antenna 1a is not used, the user may deposit the rod monopole antenna 1a in order to save space. Since the rod monopole antenna 1a needs to be drawn out to a certain length about 76 cm in use condition, the rod monopole antenna 1a is easily broken, space wasting, and deficient in esthetic appearance. On the contrary, the PE dipole antenna 2a is commonly applied to mounted stereo facilities, and may prevent from being broken. However, the price of the PE dipole antenna 2a is high, about US$1, and a long parallel transmission line 20a connecting radiating metal wires 22a, 24a of the PE dipole antenna 2a with an antenna plug set on the stereo facility is needed, which makes the PE dipole antenna 2a to be entwisted and knot.
In brevity, the rod monopole antenna 1a has a larger size and exposes on a housing of a portable or handy radio, so that the rod monopole antenna 1a is easily broken by external force, wastes space and lacks esthetic appearance. The PE dipole antenna 2a costs higher price, and is easily entwisted and knots. Architecture of these antennas not only makes users more inconvenient in use but also lacks esthetic appearance. In order to improve the above-mentioned problems of exposure antennas, it is necessary to design a hidden antenna with impedance and bandwidth suitable for requirements of the FM system.
TW patent No. M283445 discloses a mobile phone with FM antenna including a minimized hidden antenna, which is assembled on two sides and the bottom of the mobile phone. In such architecture, the antenna is often too close to the ground plane so that the antenna commonly has great capacitance and inductance Impedance matching of the antenna becomes worse, effecting signal quality of the FM antenna.
In addition, TW patent No. 200620752 discloses an antenna for mobile terminal and mobile terminal including an antenna composed of various units. The antenna has an antenna element installed on a housing and a metal coil hidden inside the housing. Such architecture often combines metal coils of the antenna with those inside the housing of the mobile phone, which increases the complexity and production cost of the antenna.
Besides, US publication patent No. 2006/0111163A1 disclose an FM transmission antenna device in a portable terminal. The FM transmission antenna device includes an antenna pattern attached to a surface of the portable terminal, an FPC extended from the antenna pattern, and a connector provided at an end of the FPC, for connecting the FPC to an FM transmitter. However, the antenna structure is complex, antenna area is small, and radiation property is bad.